In a conventional half-duplex radio communication system, there is provided an antenna switching switch for performing a connection switching, and an antenna is used in common for transmission and reception (refer to, for instance, Non-Patent Documents 1 to 4).
FIG. 8 illustrates a transmitting and receiving circuit in the conventional half-duplex radio communication system. As illustrated in FIG. 8, the conventional radio communication system conducts the communication by selectively connecting a transmitting circuit or a receiving circuit to an antenna ANT using an antenna switching switch 101.
At a time of reception, the antenna switching switch 101 is controlled and a signal received by the antenna ANT is input to a receiving circuit (a low noise amplifier (LNA) being a receiving side amplifier) 102 via a matching circuit (Rx) 103, as illustrated in FIG. 8. Further, at a time of transmission, the antenna switching switch 101 is switched to connect the antenna ANT and a transmitting circuit, and a signal output from a transmitting circuit (a power amplifier PA being a transmitting side amplifier) 104 is transmitted from the antenna ANT via a matching circuit (Tx) 105.
In the conventional radio communication system having the antenna switching switch 101, the receiving side circuit and the transmitting side circuit are not electrically connected. For this reason, each of the matching circuit (Tx) 105 between the transmitting circuit (PA) 104 and the antenna ANT and the matching circuit (Rx) 103 between the antenna ANT and the receiving circuit (LNA) 102 is individually designed.
In a weak radio communication for a communication distance of about several meters (radio communication conducted by a radio station requiring no license), a transmission power is small (for instance, the power is less than −41.3 dBm (300 MHz) in a Japanese domestic standard) and an influence of signal leakage between the transmitting circuit and the receiving circuit is small, so that the communication is applied to a radio communication system of low power consumption and small size to be incorporated in a portable device. Therefore, a reduction of a mounting area is required.
Here, since the transmission power is small in the weak radio communication, an antenna may be used in common without using an antenna switching switch (with no antenna switching switch), which enables to reduce the number of parts (mounting area). However, a transmitting side circuit and a receiving side circuit are electrically connected all the time, so that an optimum design may not be conducted with respect to matching circuits that simultaneously match between an antenna and a transmitting circuit (PA) and between the antenna and a receiving circuit (LNA), and thus it is difficult to optimize the matching circuits on a transmitting side (Tx) and on a receiving side (Rx).    Non-Patent Document 1: H. Ishida, et al. “A High-Power RF Switch IC Using AlGaN/GaN HFETs with Single-Stage Configuration”, IEEE Trans. On Electron Devices, vol. 52, no. 8, pp. 1893-1899, August 2005    Non-Patent Document 2: D. Kelly, et al. “The state-of-the-Art of Silicon-on-sapphire CMOS RF Switches”, CSIC 2005 Digest, pp. 200-203, 2005 IEEE    Non-Patent Document 3: R. Point et al, “An RF CMOS Transmitter Integrating a Power Amplifier and a Transmit/Received Switch for 802.11b Wireless Local Area Network Applications”, 2003 IEEE Radio Frequency Integrated Circuits Symposium    Non-Patent Document 4: C. K. Lee, et al, “An ISM-915 MHz RF Transceiver IC”, in 1999 Int. Symp. VLSI Technology, Systems, and Applications, June, pp. 163-166